Problem: Which of these numbers is prime? ${35,\ 41,\ 49,\ 57,\ 70}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 35 are 1, 5, 7, and 35. The factors of 41 are 1 and 41. The factors of 49 are 1, 7, and 49. The factors of 57 are 1, 3, 19, and 57. The factors of 70 are 1, 2, 5, 7, 10, 14, 35, and 70. Thus, 41 is a prime number.